Make code blocks more readable

[TrialDragon]

Why

Folks found that the inline code blocks looked weird and lacked contrast making them hard or annoying to read.

What

Fixes #2175

Uses darker colors for darkmode code block background and lighter on light mode (also made light mode background darker to let the contrast show through; this probably needs more work in a proper lightmode redesign PR). Also decreases the empty space next in the blocks so that it doesn't look like there is a space character when their isn't, especially when there is punctuation next to a code block. Also lowered the border radius and made it relative so that it isn't cutting into the letters in an ugly way.

How

Changed color variables (and linked the two separate code background colors that were the same), changed paddings and margins of content lists, and changed the global border radius variable.

Try

1. Switch into your bevy website directory
   1.opt Add my repository for the site if you haven't before: git remote add trialdragon [email protected]:TrialDragon/bevy-website.git
2. Fetch my repository for the site: git fetch trialdragon
3. Switch into the branch for this PR: git switch trialdragon/2175_improve_readability_of_code_blocks
4. Run the site: zola serve --open
   4.opt If you need to install Zola, see https://www.getzola.org/documentation/getting-started/installation/
5. Explore the site, use it, see how you feel about the styling changes.